1. Field of the Invention
The present invention relates to a method and system for monitoring the deformations of a tyre in motion.
In particular, the present invention relates to the possibility of determining the deformations of the carcassstructure of a tyre from inside the structure, for example in order to provide automatic intervention actuated or commanded by the driver, in the driving and/or control system of the vehicle, to change the vehicle's behaviour according to measurements made on the tyre.
2. Description of the Related Art
There are known prior art devices for measuring characteristic parameters such as pressure and temperature inside a tyre.
Patent application EP 887211 describes a tyre monitoring system comprising a sensor which is positioned inside the tyre and is enabled to create an electrical impulse when the said sensor passes through the footprint area formed by the contact of the tyre with the ground during rolling. The system described in this patent also comprises means for finding the ratio of the said electrical impulse to the duration of one revolution of the tyre and means for transmitting the said ratio to a processing unit within the vehicle.
In particular, the sensor is placed within the tread of the tyre in such a way that the said electrical impulse has a first peak at the entry of the sensor into the footprint area and a second peak at the exit from the footprint area.
According to the teaching of this patent, the ratio between the time elapsed between the two peaks and the period of a complete revolution makes it possible to know the extent of flattening of the tyre during the travel of the vehicle.
This is because the sensor determines the instant of entry into the footprint area and the instant of exit from the area. It is thus possible to measure the length of this area, if the angular velocity of the tyre and its radius are known. The length of the footprint area is then related to the flattening of the tyre, which is a critical parameter of the tyre in operation, particularly in tyres for heavy motor transport.
Patent application EP 689950 describes a different method for monitoring parameters of a tyre such as its pressure and temperature. In particular, a self-powered programmable electronic device positioned on the inner surface of a tyre or on its mounting rim is used. This device can be used to monitor and store the pressure, temperature and the number of rotations of the tyre, and may comprise an extensometer whose output signal measures the bending of the inner surface of the tread, or an accelerometer which measures the value of the acceleration which the tread undergoes. Additionally, the device is activated by an external radio-frequency signal and transmits an alarm signal if a predetermined limit of the measured values is exceeded.
There are also known methods for measuring deformations of the tread of a tyre during motion and for transmitting it to a receiver located on the vehicle.
Patent application WO 93.25400 describes a sensor, comprising a resonant circuit which oscillates at a predetermined characteristic frequency, located within the tread of a tyre, and is capable of transmitting a signal directly dependent on the aforesaid deformations. This resonant frequency is affected by the deformations of the tread during motion and the sensor transmits electromagnetic waves proportional to these variations of the resonant frequency. These electromagnetic waves are received by a processing unit connected to a receiver located inside the vehicle.
U.S. Pat. No. 5,247,831 describes a method for monitoring the behaviour of the footprint area of a tyre during the travel of the vehicle to optimize the driving of the vehicle. In particular, a piezoelectric sensor consisting of longitudinal strips of piezoresistive rubber is inserted into the tread. This sensor is capable of measuring the deformations of the tread, since the strips change their electrical resistance as a function of the said deformations.
There are also known methods and devices for acting on the driving and/or control system of the vehicle, particularly on the devices which control the suspension of the vehicle, to control its behaviour according to the information obtained from the axles of the vehicle or from the wheel hubs.
The publication “A method for the evaluation of the lateral stability of vehicles and tires” by the Società Pneumatici Pirelli S.p.A., presented at the International Automobile Tire Conference, Toronto, Canada, on 22 Oct. 1974, illustrated a method for determining the dynamic behaviour of the vehicle as a function of the forces developed by the various types of tyre sets and the various conditions of the road surface.
The publication principally cites three forces which determine the dynamic behaviour of a tyre, namely the vertical force, the longitudinal force and the lateral force.
The vertical force is that which is due to the dynamic load to which the tyre is subjected. The longitudinal force is that which is due to the moment applied to the axis of the tyre as a result of an acceleration or braking of the vehicle. The lateral force, also present in conditions of normal travel in rectilinear motion, is the resultant of the force due to the characteristic angles of the vehicle suspension (camber and toe-in) and to the ply-steer developed by the layers of inclined cords of the belt structure of the tyre, and of the thrust generated by the centrifugal force during drifting. The sum of the forces developed by the four tyres generates a resultant system applied to the centre of gravity of the vehicle, which balances the inertial forces and determines the attitude of the vehicle as a function of the characteristics of the suspension system of each axle. This resultant system is determined by means of suitable accelerometers, one positioned on the front axle and one on the rear axle of the vehicle. The article shows a set of graphs showing the centripetal acceleration or the centripetal force applied to the centre of gravity of the vehicle as a function of the angle of slip of the corresponding axle. From graphs plotted for different ground conditions and different types of tyres, it is possible to determine the behaviour of the vehicle provided with a predetermined set of tyres and to determine any lateral slipping of either the front or the rear axle in cornering.